Information reproducing method

ABSTRACT

An information reproducing method for reproducing information recorded on a medium by irradiating a laser beam and detecting a reflected light beam from the medium. The method includes detecting a plurality of reflected light beams in which the polarities of variations in reflected light beam intensity distributions are substantially inverted to each other, the distribution variation being produced when a light spot of the laser beam crosses a track on the medium, adding focus error signals for the plurality of reflected light beams to generate a focus error signal, and taking a difference between tracking error signals of the plurality of reflected light beams to generate a tracking error signal.

BACKGROUND OF THE INVENTION

[0001] The present invention belongs to an optical head used in anoptical disk device, and more particularly relates to a technique forenhancing a performance in detection of a position controlling signalfor an optical spot thereof.

[0002] Conventional techniques on methods for controlling a focal pointposition in an optical disk device are described in, for example,“Fundamentals and Applications of Optical Disk Storage”, Y. Tsunoda,1995, 1st edition (Korona Corp., Tokyo), pp. 79-83. According to thisliterature, there are the following methods: Foucault method (Knife edgemethod), an astigmatic method, a beam size detection method, an imagerotating method, and so on. From criteria such as simplicity of anoptical system required, the ease with which the adjustment can be made,and the ease with which combination with a tracking detection can beachieved, the most prevailing method, at the present stage, is theastigmatic method. In the astigmatic method, however, there existed aproblem that, when an optical spot crosses a track on the surface of astorage film, a disturbance is apt to occur in a focus error signal inassociation with a decentering of an optical disk. This disturbance islikely to occur especially when astigmatism takes place in a focusedspot or the optical spot is shifted on an optical detector. Examples ofmethods for reducing the disturbance are disclosed as follows: A methodof reducing the disturbance by blocking light out of a central portionof a detected light beam is disclosed in JP-A-6-162527 andJP-A-6-309687, a method of reducing it by adjusting rotation of anobjective lens is disclosed in JP-B-5-68774, and a method of reducing itby means of an operation between a light with astigmatism and a lightwithout astigmatism in a detected system is disclosed in JP-A-5-197980.None of them, however, is a fundamental method for solving theabove-described problem. Thus, at the present stage, the reducing effectobtained is not necessarily enough.

[0003] In particular, in a land-groove type optical disk employed in aDVD-RAM planned to be brought into a commercial stage soon, thedisturbance occurs quite outstandingly. The reason is as follows: In theland-groove type optical disk, a width of a guiding groove (groove) issubstantially equal to a width of a portion of a guiding inter-groove(land), and information is stored on the both sides thereof. On accountof this, a pitch of the guiding groove itself, when compared with anoptical spot, is formed to be larger than in conventional optical disks.This extraordinarily intensifies a tracking error signal according to apush-pull method described later, thus causing the disturbance to occurquite outstandingly. This condition, accordingly, brings about asituation that, in an optical head for the DVD-RAM, it can not be helpedemploying the Foucault method or the beam size detection method theconfiguration or the adjustment of which is complicated.

[0004] Conventional techniques on methods for controlling a tracking inan optical disk device are similarly described in, for example, theabove-cited “Fundamentals and Applications of Optical Disk Storage”, Y.Tsunoda, 1995, 1st edition (Korona Corp., Tokyo), pp. 83-92. Accordingto this literature, there are methods such as a three-spots method and adiffracted light differential method (push-pull method). Judging fromcriteria such as simplicity of an optical system required, the ease withwhich the adjustment can be made, and a resistance to the disturbance,the three-spots method is mainly employed in a read only type opticaldisk such as a compact disk (CD). Meanwhile, the push-pull method ismainly employed in the case of a magneto-optical disk or the DVD-RAMwhich needs a high laser emission power at the time of the recording. Atthis time, there can be considered another way in which, exchanging theroles with each other, the push-pull method is employed toward the CDand the three-spots method is employed toward writable optical disks.However, there exist circumstances which make such an employmentimpossible.

[0005] In performing a CD pick up, in order to cause a focused spot tofollow a decentering of the optical disk for the necessity of low price,the objective lens is moved by only being mounted on a lens actuator.Then, if the push-pull method is employed, it turns out that a detectedlight beam moves on the optical detector. This phenomenon appears as anoff-set. Also, at a pit depth of λ/(4n) (λ: light wavelength, n:substrate refractive index) at which a signal amplitude becomes largestin the reproduction-only type optical disk, there are the followingproblems: Of diffracted light by means of a periodic structure of trainof pits in the radial direction, 0th order light becomes smaller. Inaddition to this, even when the focused spot is off-track, no unbalanceoccurs in interference intensity between the 0th order light and ±1storder diffracted lights. This makes it impossible to obtain the trackingerror signal.

[0006] Meanwhile, in the storage-able optical disks, especially in themagneto-optical disk, compensation for the decentering of the opticaldisk is usually performed by an actuator called a coarse actuator. Thecoarse actuator mounts the optical head or only a portion of theobjective lens and an objective lens actuator so as to allow the opticalspot to come near to a proximity of a track to be objected. Namely, themagneto-optical disk is constituted in such a manner that, of a trackingerror, the low frequency components are compensated by the coarseactuator and the high frequency components are compensated by theobjective lens actuator, thereby enhancing a reliability needed for thestorage operation. Accordingly, an amount of movement by the objectivelens actuator is lower than in the read only type optical disk such asthe CD. This makes it possible to employ the push-pull method which hashigher light utilization efficiency than the three-spots method does.

[0007] Also, if the three-spots method is employed toward thestorage-able optical disks, as described on page 127 of “TechnicalDigest of Symposium on Optical Memory ‘86”, there take place thefollowing problems: First, in an optical disk such as the DVD-RAM, i.e.the type of optical disk that performs the storage by means of avariation in reflectance of a storage mark, at the time of the storageoperation, there arises a difference in the amount of light between apreceding sub-spot and a subsequent sub-spot. This causes an off-set tooccur in the tracking error signal. Also, in the case of themagneto-optical disk, there exists a feedback light back to asemiconductor laser. On account of this, a tilt of the disk unbalances acondition of stray-lights interference on the both sides of sub-spots.This also causes an off-set to occur. Moreover, as described already,the land-groove type optical disk is employed in the DVD-RAM. Thiscircumstance can also be mentioned as a reason for making it impossibleto employ the three-spots method toward the DVD-RAM. Namely, in theland-groove type optical disk, a width of the land portion is originallymade equal to that of the groove portion in order to make an amount ofreflected light of the land portion equal to that of the groove portion.This necessarily results in a fact that, even when an optical spot isoff-track, the amount of light scarcely varies, thus making itimpossible to obtain a tracking error signal according to thethree-spots method. Accordingly, it can not be helped employing thepush-pull method in the DVD-RAM. However, unlike the case of themagneto-optical disk, it is required to lower the price of the DVD-RAMdown to a price close to the price of the CD. Consequently, it becomesabsolutely necessary to reduce the off-set in a tracking error signalwhich accompanies the movement of the objective lens according to thepush-pull method.

[0008] A conventional technique for solving the above-mentioned problemin the DVD-RAM is described in, for example, “National TechnicalReport”, Vol. 40, No. 6, (1994), pp. 771-778. Here, the optical diskdevice is constituted as follows: The objective lens, a λ/4 plate, and apolarizing diffraction grating are integrally mounted on an objectivelens actuator. Moreover, the polarizing diffraction grating isconstituted so that interference regions, in which, of diffracted lightby mean of the disk, ±1st order diffracted light and −1st orderdiffracted light each interfere with 0th order light, are diffractedwith a different diffraction angle, respectively. This constitutionmakes it possible to separate, on the optical detector, the interferenceregion between the +1st order diffracted light and the 0th order lightfrom the interference region between the −1st order diffracted light andthe 0th order light. From this, the above-mentioned literature shows thefollowing: If a dual-divided optical detector is constituted so that,when the objective lens is moved, the lights do not stray out of theoptical detector, it becomes possible to eliminate the off-set caused bythe phenomenon that the optical spots move on the optical detector.

[0009] Also, employing the polarizing diffraction grating as adiffraction grating makes the following possible: When a light headingfor the disk passes through the polarizing diffraction grating, thediffraction efficiency is made substantially equal to zero, and when areflected light from the disk passes through the polarizing diffractiongrating again, the diffraction efficiency is caused to become anappropriate value. Meanwhile, in the case of a non-polarizing ordinarydiffraction grating, it diffracts the light heading for the disk, too,thus making it impossible to avoid a loss of the amount of light.Employing the polarizing diffraction grating in this way allows only thenecessary diffraction of the reflected light to occur, thus making itpossible to prevent the loss of the amount of light.

[0010] However, in this conventional example, the objective lens, theλ/4 plate, and the polarizing diffraction grating are integrally mountedon the objective lens actuator. This constitution results in a problemthat a movable portion of the actuator becomes heavy, thus restricting aresponse speed of the actuator down to a low level. Since optical disksare being improved in the storage density and at the same time arebecoming faster in the transfer rate year by year, the above-describedconventional example is not able to meet the trend of even furtherspeeding-up in the near future.

[0011] Another method, which, with no other optical component except theobjective lens mounted on the objective lens actuator, makes it possibleto eliminate the tracking error signal off-set which accompanies themovement of the objective lens according to the push-pull method, isdisclosed in the above-described “Technical Digest of Symposium onOptical Memory ‘86”, pp. 127-132. This method is referred to as adifferential push-pull method. In the method, the three-spots method isemployed, and respective tracking error signals according to thepush-pull method are subtracted on a main-spot and two sub-spots,thereby eliminating the tracking error signal off-set which accompaniesthe movement of the objective lens. Namely, in the method, the sub-spotsare located in such a manner that they are shifted on the both sides ofthe main spot by one-half of a period of the guiding groove, therebysimultaneously detecting a light beam in which variations ininterference intensity distribution of a reflected light beam reflectedfrom the disk in association with an off-track are inverted, and thusgenerating opposite phase tracking error signals which contain theoff-set in the same phase. Then, these opposite phase tracking errorsignals are subtracted, thereby allowing only the off-set to becancelled. According to this conventional example, the ratio of the gainto amplify the signal by the main spot to the gain of the signal by thesub-spot is chosen so as to compensate the intensity unbalance caused bydiffraction efficiency characteristics of the diffraction grating togenerate sub-spots. The use of this conventional example, with no otheroptical component except the objective lens mounted on the objectivelens actuator, basically makes it possible to eliminate the trackingerror signal off-set which accompanies the movement of the objectivelens according to the push-pull method. In the present conventionalexample, however, no countermeasure is taken against the mixture of thedisturbance into the focus error signal when a focused spot crosses theguiding groove in the astigmatic focus error detecting method describedearlier. Also, as described in the present conventional example, whenone of the sub-spots lies in a post-stored track and the other lies in apre-stored track, the effect of reducing the off-set is not enough.Further, although not described in the present conventional example,when a total amount of reflected light on the guiding grooves differsfrom a total amount of reflected light on the guiding inter-grooves, theoff-set also remains in the present conventional example. This situationarises when a width-of the guiding groove is not equal to that of theguiding inter-groove. However, in the case of the DVD-RAM employing theland-groove type optical disk in which the width of the guiding grooveis equal to that of the guiding inter-groove, such a situation alsoarises if the main-spot lies in the post-stored track and the twosub-spots lies in the pre-stored track or in the case opposite thereto.Still further, in the present conventional example, there exist theplurality of optical spots. This brings about a disadvantage in thelight utilization efficiency at the time of the storage.

[0012] Moreover, the gains to amplify the signals by main spot andsub-spots chosen in this conventional method is insufficient to cancelthe effect completely. The reason is as follows. As described later,when a width of the guiding groove does not substantially equal to halfof the pitch of guiding grooves, the reflectance of the light when thefocused spot is at the guiding groove is different from that when thefocused spot is at the inter-groove. It is also necessary to compensatethis unbalance of reflectance for perfect offset cancellation. For thehigher the density of the optical disk, the allowance of the offset isthe severer. Therefore this insufficient cancellation must be a problem,recently.

[0013] Still more, in this conventional example, the optical disk wasnot a land-groove type optical disk. Therefore, there is no cross-talkfrom stored information signal, because the sub-spots on the opticaldisk are not on the information track at readout process. In the case ofland-groove type optical disk such as DVD-RAM, however, the sub-spots isalso on the information track of reading out from the disk. This resultsin disturbance to the tracking error signal.

[0014] Another method, which cancels the disturbance in the focus errorsignal of astigmatic method, is disclosed in the JP-A-4-168631. Also inthis method, the main spot and sub-spots by a diffraction grating ispositioned onto the optical disk at the distances of the half of thepitch of guiding grooves in the radial direction of the disk. Thereflected beams from these focused spots pass through a cylindricallens, then detected by three quadratic photo-detectors, respectively.From the output signal of these photo detectors, three focus errorsignals are obtained by calculation in the electric circuit. These focuserror signals are amplified with gains which are proportional to thereciprocals of the light intensity of each focused spot on the opticaldisk, which are not proportional to the reciprocal of the reflectedlight intensity. Then summation of these amplified focus error signalsis calculated in the electric circuit. Employing this method, the extradisturbance to the conventional focus error signal by aberrations ormiss-alignment of the optical components or photo detector can beeliminated. The optimum gains for disturbance cancellation for thismethod is different from those for differential push-pull method asmentioned. However, in this method, no tracking method is disclosed.Further more, if the differential push-pull method described in theconventional literature itself is employed with this focusing errordetection method, it is necessary to set the gains to amplify the signalby each reflected light beam equal between in the focus error signal andin the tracking error signal, namely proportional to the reciprocals ofthe light intensity of incident focused spots on to the optical disk. Itresults in the insufficient cancellation of the offset of tracking errorsignal as mentioned.

SUMMARY OF THE INVENTION

[0015] In view of the above-described conventional techniques, in themethod and the device for detecting the focal point shift, a problem tobe solved by the present invention is to fundamentally eliminate thedisturbance which occurs in the focus error signal in association withthe decentering of an optical disk when an optical spot crosses a trackon the surface of the storage film.

[0016] Also, another problem to be solved by the present invention is tofundamentally cancel the off-set which occurs simultaneously in thetracking error signal in association with the movement of the lens.

[0017] Also, when employing a method such as the differential push-pullmethod in which a light beam, in which variations in interferenceintensity distribution of a reflected light beam at the time when anoptical spot on the disk crosses the guiding groove are inverted, isgenerated simultaneously with the ordinary light beam and thus theopposite phase tracking error signals which contain off-set componentswith the same phase are generated so as to cause the same phase off-setto be cancelled, another problem to be solved by the present inventionis to cancel an off-set which occurs from a difference in the totalamount of reflected light between these light beams.

[0018] Also, another problem to be solved by the present invention isnot only to cancel, in the differential push-pull method, the off-setwhich occurs in the tracking error signal in association with themovement of the lens but also to fundamentally eliminate, in the focalpoint shift detecting method, the disturbance which occurs in the focuserror signal in association with the decentering of an optical disk whenan optical spot crosses a track on the surface of the storage film.

[0019] Also, another problem to be solved by the present invention is toobtain, with the sub-spots in the differential push-pull method locatedon the same track as the main-spot, the same effect of canceling thetracking error signal off-set which accompanies the movement of theobjective lens.

[0020] Also, another problem to be solved by the present invention is toconstitute the optical disk device so that a single spot on the diskexhibits the same effect as the differential push-pull method does.

[0021] Also, another problem to be solved by the present invention is toobtain these effects toward the astigmatic focal point shift detectingmethod and the push-pull tracking detecting method in particular.

[0022] Also, another problem to be solved by the present invention is toillustrate a configuration of an optical detector which allows theseeffects to be obtained.

[0023] Also, another problem to be solved by the present invention is toenhance performance in the canceling of the tracking error signaloff-set due to the movement of the objective lens when combining thedifferential push-pull method with the additive astigmatic method.

[0024] Also, another problem to be solved by the present invention is toeliminate an influence of the disturbance due to the information pitswhen combining the differential push-pull method with the additiveastigmatic method so as to apply them together to the land-groove typeoptical disk.

[0025] In order to solve the above-described problems, an optical headcomprises at least a semiconductor laser, a light-converging opticalsystem for converging an emitted light from the semiconductor laser ontoan optical disk having a periodic structure in a radial direction as atleast one focused spot, an optical detection system for detecting areflected light from the optical disk, and an electrical circuit forcalculating an amount of received light through a photoelectricconversion thereof so as to obtain at least one of a focus error signalof the focused spot converged on the optical disk, a tracking errorsignal of the focused spot converged on the optical disk, and a datasignal stored in the optical disk. The light-converging optical systemincludes means for generating a plurality of reflected light beams inwhich polarities of their intensity distribution variations at the timewhen the periodic structure crosses the focused spot on the optical diskare substantially inverted with each other, the optical detection systemincludes means for splitting and simultaneously detecting the pluralityof reflected lights, and the electrical circuit obtains the focus errorsignal by taking summation of focus error signals of the respectivereflected light beams so that variations of the focus error signalcaused by their intensity distribution variations cancel out with eachother.

[0026] Also, at this time, a difference between respective trackingerror signals of the plurality of reflected lights the polarities ofwhich are inverted with each other is simultaneously defined as thetracking error signal.

[0027] Moreover, at this time, in defining, as the tracking errorsignal, the difference between the respective tracking error signals ofthe plurality of reflected light beams the polarities of which areinverted with each other, in the electrical circuit, the respectivetracking error signals are amplified with a gain which is proportionalto a ratio between reciprocals of respective total amounts of thereflected lights when one of said focused spot is at the informationtrack of said optical disk, and after that a difference between therespective tracking error signals thus amplified is calculated, thenbeing defined as the tracking error signal.

[0028] Also, in these constitutions, there is provided a beam splittingdevice for splitting the reflected light beam from the optical disk offfrom an optical path extending from the semiconductor laser, and themeans for generating said plurality of reflected light beams thepolarities of intensity distribution variations of which aresubstantially inverted with each other is a diffraction grating locatedbetween the semiconductor laser and the beam splitting device. Moreover,the diffraction grating is installed in such a manner as to be tiltedtoward the radial direction of the optical disk so that focused spots of±1st order diffracted lights by means of the diffraction grating arelocated in such a manner that, on the optical disk and with reference toa focused spot of a 0th order light, they are shifted by about one-halfof a period of the above-described periodic structure in oppositedirections to each other in the radial direction.

[0029] Also, there is provided a beam splitting device for splitting thereflected light beam from the optical disk off from an optical pathextending from the semiconductor laser, and the means for generating theplurality of reflected light beams the polarities of intensitydistribution variations of which are substantially inverted with eachother is a diffraction grating located between the semiconductor laserand the beam splitting device. Moreover, the diffraction grating hasgratings the phase of which is inverted at an interval of substantiallyλD/(2NA·P) (λ: light wavelength, NA: numerical aperture of an objectivelens, P: period of the periodic structure in the radial direction on theoptical disk, D: effective light beam diameter on the diffractiongrating) in regions of a common width in the radial direction on theoptical disk. The diffraction grating is installed in such a manner thata direction of the gratings thereof is in parallel to a tangentialdirection of the optical disk so that, on the optical disk, focusedspots of ±1st order diffracted lights by means of the diffractiongrating are located on the same track as a focused spot of a 0th orderlight. Furthermore, the optical detection system splits and detectsthese focused spots. Then, a data signal is obtained from an amount ofreceived light signal resulting from the 0th order light.

[0030] Still further, there is provided a beam splitting device forsplitting the reflected light beam from the optical disk off from anoptical path extending from the semiconductor laser, and the means forgenerating the plurality of reflected light beams the polarities ofintensity distribution variations of which are substantially invertedwith each other is a polarizing phase shifter located between thesemiconductor laser and the beam splitting device. The polarizing phaseshifter is constituted so that it relatively inverts a phase of alinearly polarized light component, which is polarized in a specificdirection, at an interval of substantially λD/(2NA·P) (λ: lightwavelength, NA: numerical aperture of an objective lens, P: period ofthe periodic structure on the optical disk, D: effective light beamdiameter on a diffraction grating) in regions of a common width in theradial direction on the optical disk, and a phase of a linearlypolarized light component perpendicular to the linearly polarized lightcomponent is not varied over a whole system of the polarizing phaseshifter. Furthermore, the optical detection system splits and detectsthese polarized light components with the use of a polarizing beamsplitting device. Then, a data signal is obtained from the polarizedlight component to which no phase inversion is added.

[0031] In particular, the above-described constitutions are embodied byemploying the astigmatic method for the focus error detection and byemploying the push-pull method for the tracking error detection.

[0032] Also, in the optical detection system, there is provided anoptical detector in which there exist at least two sets of opticaldetection regions each of which receives a single optical spot with afour-divided optical detection region.

[0033] Also, an optical head includes a semiconductor laser, alight-focusing optical system for focusing, as at least one focusedspot, an emitted light from the semiconductor laser onto an optical diskwhich has a periodic structure such as guiding grooves in its radialdirection, an optical detection system for detecting a reflected lightfrom the optical disk, and an electrical circuit for obtaining, from thereflected light, both a focus error signal of one of the focused spotsand a tracking error signal thereof. In the optical head, sub-spots, forexample, are located by an apparatus such as a diffraction grating insuch a manner that they are shifted from a main-spot by one-half of aperiod of the guiding grooves, thereby generating two kinds of and, foreach of the kinds, at least one or more of reflected light beams inwhich polarities of their intensity distribution variations at the timewhen the periodic structure crosses the focused spots are substantiallyinverted with each other. The optical detection system splits anddetects the plurality of reflected light beams. In the electricalcircuit, focus error signals, which are obtained by each adding focuserror signals generated from the two kinds of and, for each of thekinds, at least one of reflected light beams, are amplified and addedfurther, thereby obtaining the focus error signal. Moreover, trackingerror signals, which are obtained by each adding tracking error signalsgenerated from the two kinds of and, for each of the kinds, at least oneof reflected light beams, are amplified and subtracted from each other,thereby obtaining the tracking error signal. At this time, an opticaldisk such as the land-groove type optical disk is used in which theguiding grooves constitute the periodic structure and, as compared withan occasion when one of the focused spots is situated at a guidinggroove, an error of the reflectance on an occasion when it is situatedat a guiding inter-groove falls within a range of ±10% thereof. The useof such type of optical disk makes it possible to cause an amplificationgain ratio between the tracking error signals of the two kinds ofreflected light beams to coincide with an amplification gain ratiobetween the focus error signals of the two kinds of reflected lightbeams.

[0034] Also, in a similar optical head, in a case where the optical diskused is an optical disk other than the land-groove type optical disk,i.e., in a case where the reflectances differ between an occasion whenone of the focused spots is positioned on an information track of theoptical disk and an occasion when it is positioned at a position whichis apart from the information track by one-half of the period of theperiodic structure, it turns out that the amplification gain ratiobetween the tracking error signals of the plurality of reflected lightbeams differs from the amplification gain ratio between the focus errorsignals of the plurality of reflected light beams.

[0035] Also, in the optical head, the electrical circuit for detectingthe sub-spots is provided with a frequency characteristic which makes itpossible to cut off a frequency bandwidth of a read-out signal ofrecorded information written in the optical disk.

[0036] Other objects, features and advantages of the present inventionwill become apparent from the following detailed description of theembodiments of the invention taken in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0037]FIG. 1 is a diagram for showing a constitution of an opticalsystem in a basic embodiment of the present invention;

[0038]FIG. 2 is a diagram for showing locations of focused spots on anoptical disk and intensity distributions of reflected light beams atthat time;

[0039]FIG. 3 is a diagram for showing a circuit calculation method of anoutput of a detector;

[0040]FIG. 4 is a diagram for showing a calculation method of the outputof the detector;

[0041]FIG. 5 is a diagram for explaining a tracking error signal off-setdue to movement of an objective lens;

[0042]FIG. 6 is a diagram for explaining an disturbance into a focuserror signal due to astigmatism;

[0043]FIG. 7 is a diagram for showing a constitution of an opticalsystem in an embodiment in which a phase-inverted diffraction grating isemployed;

[0044]FIG. 8 is a diagram for explaining a detailed structure of thephase-inverted diffraction grating;

[0045]FIG. 9 is a diagram for explaining a manner in which phase shiftregions are overlapped in diffraction of a phase-inverted light by meansof optical disk guiding grooves;

[0046]FIG. 10 is a diagram for showing interference phase differencesadded to an optical disk diffracted light by means of the phase-inverteddiffraction grating;

[0047]FIG. 11 is a diagram for explaining a manner in which, when themovement of the objective lens exists, phase shift regions areoverlapped in diffraction of a phase-inverted light by means of opticaldisk guiding grooves;

[0048]FIG. 12 is a diagram for showing a constitution of an opticalsystem in an embodiment of the present invention in which a polarizingphase shifter is employed;

[0049]FIG. 13 is a diagram for explaining the principle of thepolarizing phase shifter;

[0050]FIG. 14 is a diagram for explaining calculation of an disturbancedue to an ordinary crossing over a guiding groove by means of a focuserror signal;

[0051]FIG. 15 is a diagram for explaining calculation of an disturbancedue to a crossing over a guiding groove by means of a focus error signalat the time of employing the phase-inverted diffraction grating;

[0052]FIG. 16 is a diagram for explaining calculation of an disturbancedue to a crossing over a guiding groove by means of a focus error signalin a differential push-pull method;

[0053]FIG. 17 is a diagram for explaining lens shift characteristics ofa tracking error signal with the use of an ordinary detecting light beamfor a focus error signal;

[0054]FIG. 18 is a diagram for explaining lens shift characteristics ofa tracking error signal with the use of a detecting light beam for afocus error signal at the time of employing the phase-inverteddiffraction grating;

[0055]FIG. 19 is a diagram for explaining lens shift characteristics ofa tracking error signal with the use of a detecting light beam for afocus error signal in the differential push-pull method;

[0056]FIG. 20 is a diagram for showing an embodiment of an opticalsystem constitution according to the present invention in which thereproduction is possible in a DVD, a DVD-RAM, a CD, and a CD-R;

[0057]FIG. 21 is a diagram for explaining details of a constitution forthe detection in the embodiment in FIG. 20;

[0058]FIG. 22 is a diagram showing a modified example of the circuitcalculation method of the detector output in FIG. 3; and

[0059]FIG. 23 is a characteristic diagram representing a frequencycharacteristic of a gain of an amplifier and a read-out signal intensityof the detector.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0060] The description will be given below concerning embodiments of thepresent invention, using the accompanying drawings.

[0061]FIG. 1 is a diagram for showing a constitution of an opticalsystem in a basic embodiment of the present invention. A light beamemitted from a semiconductor laser 101 produces a diffracted light bypassing through a diffraction grating 102. The diffracted light, by wayof a beam splitter 103, a triangle reflection prism 104 and an objectivelens 106, forms a main-spot 108 of a 0th order light and two sub-spots109, 110 of ±1st order diffracted lights on an optical disk 107. Areflected light beam, by way of the objective lens 106 and the trianglereflection prism 104 again, is reflected at the beam splitter 103. Then,the reflected light beam is provided with an astigmatism for detecting afocal point shift by a cylindrical lens 111, thus being received by anoptical detector 115. The optical detector 115 is separated into a 0thorder light four-divided optical detection region 112 and ±1st orderdiffracted lights four-divided optical detection regions 113, 114. Thetwo kinds of optical detection regions are independent of each other inperforming the detection. Here, the diffraction grating 102 is locatedin such a manner as to be tilted to some extent so that the ±1st orderdiffracted lights on the optical disk are located in such a manner as tobe shifted on the both sides of the 0th order light by one-half of aguiding groove pitch.

[0062]FIG. 2 is a diagram for showing locations of optical spots on theoptical disk and intensity distributions of reflected light beams atthat time. FIG. 2 shows a case in which a 0th order light 201 and ±1storder diffracted lights 202, 203 are shifted slightly on the left sidewith reference to groove portions 204 and land portions 205, a case inwhich they are just on-track, and a case in which they are shiftedslightly on the right side. At this time, intensity variations of a 0thorder light-detected light beam 206 and a ±1st order diffractedlights-detected light beam 207, as illustrated in FIG. 2, are shifted indirections opposite to the directions of the above-mentionedtrack-shifts of the optical spots on the disk. This is because the ±1storder diffracted lights 202, 203 on the disk are located in such amanner that they are sifted with reference to the 0th order light 201 byone-half of the track pitch. There occur such light intensitydistributions of the detected light beams in correspondence withpositions of the optical spots on the optical disk. As described in, forexample, the literatures cited earlier, this knowledge itself has beenknown publicly.

[0063]FIG. 3 is a diagram for showing a circuit calculation method ofoutputs of the optical detector. Incidentally, on the optical detector115, the intensity distributions of the reflected light beams arerotated by 90 degrees because of the astigmatism for detecting a focalpoint shift or focus error. Here, a focus error signal (AF signal) isobtained by adding components in the same diagonal direction ofcorresponding divided outputs of the divided optical detector 112 forthe main-spot and the divided optical detectors 113, 114 for thesub-spots, and then by calculating the differential signal thereof withthe use of a differential amplifier 303. Calculating the focus errorsignal in this way allows only the disturbance to be canceled outbecause, when the focused spots cross a guiding groove on the disk,variations in intensity distribution of the sub-spots are inverted witha variation in intensity distribution of the main-spot. At this time,usually, an amount of light of the sub-spots is made smaller than thatof the main-spot. Accordingly, the calculation is performed after signaloutputs of the sub-spots are amplified by an amplifier 301 by an amountcorresponding to the ratio between the amount of light of the sub-spotsand that of the main-spot. In this embodiment, however, there exist thetwo sub-spots. Assuming an intensity of the main-spot as A and that ofthe sub-spot as B, the gain on each sub-spot, actually may take a valueobtained by multiplying, by A/(2B), amplification gains of the signalsby the two sub-spots with reference to the main-spot. Meanwhile, atracking error signal (TR signal) is obtained by first adding,alternately between in the main-spot and in the sub-spots, an output forevery two regions divided into the right and left in Fig, 3, and then bycalculating the differential output thereof with the use of adifferential amplifier 304. Calculating the tracking error signal inthis way makes it possible to obtain a tracking error signal in whichonly the off-set components are canceled out, because, when the focusedspots cross the guiding groove on the disk, the variations in intensitydistribution of the sub-spots are inverted with the variation inintensity distribution of the main-spot and in addition the off-set dueto the lens shift is not inverted. Here, from the above-describedoriginal location, when the main-spot is situated on a land portion, thesub-spots are situated on groove portions. This, when a width of theland portion is different from that of the groove portion, results in adifference in the amount of reflected light between the main-spot andthe sub-spots, thereby making the off-set canceling insufficient. Insuch a case, signals of the sub-spots are amplified by an amplifier 302so that the difference in the amount of reflected light therebetween iscompensated. For example, when the information tracks exist on the landportions, the again of the amplifier 302 of the sub-spots, may take a/bwhere a represents a reflectance of the land portion as and b representsthat of the groove portion. Also, in some cases, the output of themain-spot may be lower. In such a case, the main-spot, on the contrary,is amplified. Otherwise, the gain of the amplifier 302 is made equal to1 or less. The above-described calculation method makes it possible tosimultaneously obtain the tracking error signal without the off-set dueto the lens shift and the focus error signal without the disturbance atthe time of crossing the guiding groove. In the mean time, concerning areproducing signal, a total amount of light of the main-spot isoutputted using a differential amplifier 305. Incidentally, here, theoptical disk employed is assumed to be an optical disk such as areproduction-only type optical disk or a phase-varied type optical diskwhich allows a signal to be reproduced with the use of the amount ofreflected light. However, in the case of the magneto-optical disk aswell, there exists no other difference except a difference which resultsfrom defining the data signal as a differential signal between twosignal outputs in which the polarized light components are split.Consequently, it is possible to detect the focus error signal and thetracking error signal with the use of the present embodiment.

[0064]FIG. 4 is a diagram for summarizing the calculation method at thistime. As a conclusion, what should be done is just to perform thecalculations as illustrated in FIG. 4 toward four outputs a, b, c, d ofthe 0th order light four-divided optical detection region 112 andrespective four outputs e, f, g, h, i, j, k, 1 of the 1st orderdiffracted lights four-divided optical detection regions 113, 114.Incidentally, here, a reference note RF denotes a data signal, AF afocal point shift error signal, and TR a tracking error signal.

[0065] The above-described embodiment has generally assumed the case inwhich the reflectances differ between an occasion when a focused spot issituated at the guiding groove and an occasion when it is situated atthe guiding inter-groove. However, in the land-groove type optical diskused in the DVD-RAM disk, the width of the guiding groove issubstantially equal to one-half of the track pitch, and thus thereflectances substantially equal between the occasion when a focusedspot is at the groove portion and the one when it is at the landportion. This, by omitting the amplifier 302 in FIG. 3, makes itpossible to simplify the circuit configuration as illustrated in FIG.22. Incidentally, even in the land-groove type optical disk, because ofthe fabricating error, a difference in the reflectance in the landportion toward the groove portion can be about, at the maximum, ±10%.Concerning the difference of this extent, however, the computersimulation has demonstrated the following: When an effective diameter ofthe objective lens is set to be 4 mm, even if the lens shift is 0.4 mm,the track off-set turns out to be about 0.01 μm in a DVD-RAM disk thegroove pitch of which is 1.48 μm. This means that, in the configurationin FIG. 22 as well, the track off-set is allowable. Conversely, providedthat the allowable value of the track off-set is equal to 0.05 μm, thedifference in the reflectance in the land portion toward the grooveportion has been found to be about 1.6 times under the same conditions.This means that, in an ordinary optical disk other than the land-groovetype optical disk, this value becomes more than two times greater.Accordingly, the configuration in FIG. 22, after all, can be applicableonly to the land-groove type optical disk.

[0066] Also, in the land-groove type optical disk, there exist theinformation tracks at the guiding grooves as well as at the guidinginter-grooves. Consequently, when the main-spot is situated on aninformation track, the sub-spots, naturally, are situated on theadjacent tracks. At this time, there occurs a mixture of recordedinformation into the sub-spots, which has not been assumed except forthe case of the land-groove type optical disk. In order to avoid thisphenomenon, it is appropriate to let an amplifier 301 in FIG. 22 have afrequency characteristic as illustrated in FIG. 23. In FIG. 23, ahorizontal axis in the coordinate indicates the frequency, a verticalaxis on the left side indicates the frequency characteristic of a gainof the amplifier, and a vertical axis on the right side indicates anintensity of a read-out signal on information tracks in a detector.Although the read-out signal lies in a higher frequency bandwidth ascompared with control signals such as the tracking error signal or thefocus error signal, a signal actually detected by the detector is a oneresulting from synthesizing these signals. Here, by letting theamplifier have a characteristic that the gain becomes lower in theread-out signal bandwidth, it is possible to obtain a control signalwithout the disturbance.

[0067] The optical disk in the embodiment mentioned upper was notspecialized to land-groove type optical disk. In the case of land-groovetype optical disk such as DVD-RAM, the reflectance of the light issubstantially equal when between the focused spot on the optical disk isin groove and in inter-groove. Therefore the amplifier 302 in the FIG. 3can be omitted as in FIG. 22. Of course, even in the land-groove disk,these reflectances have some error approximately 10%. However, it isexamined by computer simulation that this amount of deviation isallowable.

[0068] In FIG. 23, the frequency spectrum of the readout signal, servocontrol signal, and frequency transfer characteristics of the amplifiersin FIG. 22 is shown. The amplifier has the frequency transfercharacteristics to substantially cut off the frequency band of storedinformation signal in the optical disk.

[0069] Next, an analytical explanation will be given below concerningthe reason why such a calculation method makes it possible to cancel thetracking error signal off-set due to the movement of the objective lens.According to “Journal of Optical Society of America”, 1979, Vol. 69,No.1, pp.4-24, distribution of a reflected light beam by means of theperiodic structure of the optical disk is obtained as follows: In scalardiffraction approximation, products of Rm, i.e. m-th order Fouriercoefficients of reflectance distribution of the optical disk, and a (x,y), i.e. complex amplitude distribution of an incident light beam, areshifted by a quantity of mNA/Pλ (NA: numerical aperture, P: period ofguiding grooves, λ: wavelength), i.e. distribution shift due to a m-thorder diffraction, and, after being multiplied by exp (i2 πmu_(o)/P),i.e. phase component based on a spot position of the main spot u_(o),are added, thus obtaining the distribution. Namely, the followingEquation (1) is obtained: $\begin{matrix}{{{a^{\prime}\left( {x,y} \right)} = {\sum\limits_{m}{R_{m}{a\left( {{x - \frac{m\quad {NA}}{P\quad \lambda}},y} \right)}^{i\quad 2\pi \frac{m}{P}u_{0}}}}}{{a\left( {x,y} \right)}:\quad {{Complex}\quad {amplitude}\quad {distribution}\quad {of}\quad {incident}\quad {light}}}\quad \quad {P:\quad {{Period}\quad {of}\quad {guiding}\quad {grooves}}}\quad {\lambda:\quad {Wavelength}}\quad {{NA}:\quad {{Numerical}\quad {aperture}}}\quad {m:\quad {{Order}\quad {of}\quad {diffraction}\quad {by}\quad {grooves}}}\quad {u_{0}:\quad {{Spot}\quad {position}\quad {in}\quad {radial}\quad {direction}}}} & (1)\end{matrix}$

[0070] Here, Rm, which corresponds to a complex amplitude of a m-thorder diffracted light at the time when a parallel light beam with anamplitude 1 is launched into the optical disk at an angle perpendicularthereto, is represented by Equation (2): $\begin{matrix}{{{{R_{m} = {\frac{1}{P}{\int_{{- P}/2}^{P/2}{{R(u)}^{{- i}\quad 2\pi \frac{m}{P}u}\quad {u}}}}}\quad u:\quad {{Radial}\quad {coordinate}\quad {on}\quad {the}\quad {disk}}}{{R(u)}:\quad {{Distribution}\quad {of}\quad {complex}\quad {amplitude}\quad {reflectance}\quad {of}\quad {disk}\quad {surface}}}}\quad} & (2)\end{matrix}$

[0071] , and, in particular, in the case of rectangular grooves with awidth w and a groove depth d normalized by the wavelength, Rm isrepresented by Equation (3): $\begin{matrix}{{R_{m} = {{{sinc}\quad m} - {\frac{w}{P}\left( {1 - ^{- {{i4}\pi d}}} \right){sinc}\frac{m\quad w}{P}}}}{{w:\quad {{Guiding}\quad {groove}\quad {width}}}d:\quad {{Groove}\quad {depth}\quad {normalized}\quad {by}\quad {wavelength}}}} & (3)\end{matrix}$

[0072] Incidentally, here, sinc X has the relation 5 expressed byEquation (4): $\begin{matrix}{{{sinc}\quad x} = \left\{ \begin{matrix}\frac{\sin \quad \pi \quad x}{\pi \quad x} & \left( {x \neq 0} \right) \\1 & \left( {x = 0} \right)\end{matrix} \right.} & (4)\end{matrix}$

[0073] Using these equations, and provided that the incident light beamhas no aberration and the amplitude is uniform within an objective lenspupil surface, interference intensities between the 0th order light andthe ±1st order diffracted lights by means of the periodic guidinggrooves in the optical disk are represented by Equation (5):$\begin{matrix}{{I_{0,{\pm 1}}\left( {x,y} \right)} = {{R_{0}}^{2} + {R_{\pm 1}}^{2} + {2{R_{0}}{R_{\pm 1}}{\cos \left( {\phi \mp {\frac{2\quad \pi}{P}u_{0}}} \right)}}}} & (5)\end{matrix}$

[0074] Incidentally, here, ø has the relation expressed by Equation (6):

φ=arg(R _(±1))−arg (R ₀)  (6)

[0075] Using these equations, a tracking error signal TR according to apush-pull method at the time when there exists no movement of the lensis represented by Equation (7): $\begin{matrix}{{{\begin{matrix}{{TR} = \quad {S\left( {I_{0,{+ 1}} - I_{0,{- 1}}} \right)}} \\{= \quad {4\quad S{R_{0}}{R_{\pm 1}}\sin \quad \phi \quad \sin \quad 2\pi \frac{u_{0}}{P}}} \\{= \quad {4\quad S\frac{w}{P^{2}}{sinc}\frac{w}{P}{sinc}\quad 4\pi \quad d\quad \sin \quad 2\pi \frac{u_{0}}{P}}}\end{matrix}S:\quad {{{Area}\quad {size}\quad {of}\quad {interference}\quad {region}\quad {of}\quad 0{th}\quad {and}}\quad \pm {1{st}}}}\quad}\quad {order}\quad {diffraction}} & (7)\end{matrix}$

[0076] S:Area size of interference region of 0th and ±1st orderdiffraction

[0077] Here, as shown in FIG. 5, assuming that an optical spot 502 on adual-divided optical detector 501 is moved by the movement of theobjective lens, from an increase or a decrease in the amount of receivedlight in each divided region of the dual-divided optical detector 501, atracking error signal TR according to the ordinary push-pull method isrepresented by Equation (8), using parameters α (0<α<1), β(0<β<1)attributed to the movement of the objective lens:

[0078] TR(α, β)=S(I _(0,+1) +αI _(0, −1)+βI₀−(1−α) I _(0, −1)−(1−β)I₀)=TR(0,0)+2S(αI _(0,−1) +βI ₀)  (8)

[0079] The second term in the right hand side corresponds to theoff-set. Here, in Equation (5), if, for example, the sub-spots by meansof the diffraction grating are shifted by one-half of the track pitch, aphase inside cos in the third term in the right hand side is shifted byπ. Accordingly, at this time, I′₀, ±1 (x, y), i.e. interferenceintensities of the sub-spots, are represented with reference to u_(o),i.e. the spot position of the main spot, by Equation (9):$\begin{matrix}{{I_{0,{\pm 1}}^{\prime}\left( {x,y} \right)} = {{R_{0}}^{2} + {R_{\pm 1}}^{2} - {2{R_{0}}{R_{\pm 1}}{\cos \left( {\phi \mp {\frac{2\quad \pi}{P}u_{0}}} \right)}}}} & (9)\end{matrix}$

[0080] Moreover, in Equation (7), too, if the sub-spots are shifted byone-half of the track pitch, the tracking error signal is inverted, too.Accordingly, TR′ (α, β), i.e. a tracking error signal of the sub-spotsat the time when there exists the movement of the lens, is representedby Equation (10):

TR′(α, β)=−TR(0,0)+2S(αI′ _(-,−1) +βI ₀)  (10)

[0081] Consequently, by subtracting the tracking error signal of thesub-spots from the tracking error signal of the main-spot, a signalexpressed by Equation (11) is obtained: $\begin{matrix}\begin{matrix}{{{{TR}\left( {\alpha,\beta} \right)} - {{TR}^{\prime}\left( {\alpha,\beta} \right)}} = \quad {{2\quad {{TR}\left( {0,0} \right)}} + {2\quad S\quad {\alpha \left( {I_{0,{- 1}} - I_{0,{- 1}}^{\prime}} \right)}}}} \\{= \quad {{2\quad {{TR}\left( {0,0} \right)}} + {4\quad S\quad \alpha {R_{0}}{R_{\pm 1}}\cos}}} \\{\quad \left( {\phi + {\frac{2\quad \pi}{P}u_{0}}} \right)}\end{matrix} & (11)\end{matrix}$

[0082] Accordingly, when an off-track of the main-spot is equal to 0,namely, u_(o)=0, the off-set is represented by the following Equation(12): $\begin{matrix}\begin{matrix}{{Offset} = \quad {4\quad S\quad \alpha {R_{0}}{R_{\pm 1}}\cos \quad \phi}} \\{= \quad {4\quad S\quad \alpha \frac{w}{P^{2}}{sinc}\frac{w}{P}\left( {{2\frac{w}{P}} - 1} \right)\left( {1 - {\cos \quad 4\pi \quad d}} \right)}}\end{matrix} & (12)\end{matrix}$

[0083] Consequently, if the width of the groove is not one-half of thetrack pitch, the off-set remains. This is caused by the fact that, asseen from the second term in the right hand side in the upper stage ofEquation (11), when the main-spot is on-track, the interferenceintensity thereof differs from interference intensities of thesub-spots. Thus, by anticipating this intensity variation and setting inadvance a gain G₂ shown in FIG. 4, it is possible to cancel the off-set.Also, in the case of an optical disk such as the DVD-RAM in which theland-groove type optical disk is employed, the off-set is canceled outautomatically without setting such a gain.

[0084] The above-described description has been given concerning theeffect of canceling the off-set in the tracking error signal. The methodemployed therefor is that a light beam the interference phase of whichis inverted is detected simultaneously. By the way, this method alsocancels out the disturbance into the focus error signal when an opticalspot crosses the guiding groove, which appears as a serious problem inthe astigmatic focal point shift detecting method. The principle thereofwill be explained below: First of all, there exist two major causesconcerning the above-mentioned disturbance in association with thecrossing over the track in the astigmatic focal point shift detection.One is astigmatism exerted upon the optical spot on the disk. The otheris a shift in the four-divided optical detector. Here, the explanationwill be given by employing, as the example, a mixture of the disturbancecaused by the astigmatism. Using W₂₂, i.e. an aberration coefficient ofastigmatism, and Φ, i.e. direction angle of astigmatism, a wave surfacehaving the astigmatism is represented by Equation (13):

W(ρ, θ)=W ₂₂ρ² cos 2(θ−Φ)  (13)

[0085] ρ: Normalized radial coordinate in the pupil

[0086] θ: Polar angle coordinate in the pupil

[0087] W₂₂: Aberration coefficient of astigmatism

[0088] Φ: Direction angle of astigmatism

[0089] This can be rewritten into the Equation (14), using x, ycoordinates of an effective diameter in the pupil:

W(x, y)=W ₂₂ {(x ² −y ²)cos 2Φ+2xy sin 2Φ)  (14)

[0090] X,y : Normalized Cartesian (effective diameter) (14) coordinatesin the pupil

[0091] Accordingly, assuming that the wave surface having theastigmatism is diffracted by the optical disk and the 0th order lightand the ±1st order diffracted lights thereof are shifted by ±δ and thenare overlapped with each other on the objective lens pupil surface, aphase difference in the interference added by the astigmatism can beapproximated as a form of Equation (15): $\begin{matrix}\begin{matrix}{{\Delta \quad W} = \quad {{W\left( {{x \pm \delta},y} \right)} - {W\left( {x,y} \right)}}} \\{\cong \quad {{\pm \frac{\partial W}{\partial x}}\delta}} \\{= \quad {{\pm 2}\quad W_{22}\quad \left( {{x\quad \cos \quad 2\varphi} + {y\quad \sin \quad 2\varphi}} \right)\delta}}\end{matrix} & (15)\end{matrix}$

[0092] Then, using this equation, interference intensities between the0th order light and the ±1st order diffracted lights are represented byEquation (16): $\begin{matrix}{{I_{0,{\pm 1}}\left( {x,y} \right)} = {{R_{0}}^{2} + {R_{\pm 1}}^{2} + {2{R_{0}}{R_{\pm 1}}{\cos \left( {{\Delta \quad W} + {\phi \mp {\frac{2\pi}{P}u_{0}}}} \right)}}}} & (16)\end{matrix}$

[0093] Here, as illustrated in FIG. 6, if representative points A, B, C,D are picked up in a reflected light beam 602, which has astigmatism andis reflected from the optical disk, interference intensities at thesepoints are represented by Equations (17) to (20), using Equation (16):$\begin{matrix}{I_{A} = {C + {\alpha \quad \cos \left\{ {{W_{22}{\delta \left( {{\cos \quad 2\varphi} + {\sin \quad 2\varphi}} \right)}} + \phi - {\frac{2\pi}{P}u_{0}}} \right\}}}} & (17) \\{I_{B} = {C + {\alpha \quad \cos \left\{ {{W_{22}{\delta \left( {{\cos \quad 2\varphi} - {\sin \quad 2\varphi}} \right)}} + \phi + {\frac{2\pi}{P}u_{0}}} \right\}}}} & (18) \\{I_{C} = {C + {\alpha \quad \cos \left\{ {{W_{22}{\delta \left( {{\cos \quad 2\varphi} + {\sin \quad 2\varphi}} \right)}} + \phi + {\frac{2\pi}{P}u_{0}}} \right\}}}} & (19) \\{I_{D} = {C + {\alpha \quad \cos \left\{ {{W_{22}{\delta \left( {{\cos \quad 2\varphi} - {\sin \quad 2\varphi}} \right)}} + \phi - {\frac{2\pi}{P}u_{0}}} \right\}}}} & (20)\end{matrix}$

[0094] Assuming that, basically, these intensities appear without beingvaried on the detectors for detecting the focus error, an disturbancewhich, as shown in Equation (21), $\begin{matrix}\begin{matrix}{{AF} = {\left( {I_{A} + I_{C}} \right) - \left( {I_{B} + I_{D}} \right)}} \\{= {{- 2}{{\alpha sin}\left( {{W_{22}{\delta cos2\varphi}} + \phi} \right)}{\sin \left( {W_{22}{\delta sin2\varphi}} \right)}{\cos \left( {\frac{2\pi}{P}u_{0}} \right)}}}\end{matrix} & (21)\end{matrix}$

[0095] is cos waveform-like in shape with reference to the off-tracku_(o) is mixed into the focus error signal. Here, a focused spot, inwhich variations in intensity distribution of a reflected light beamreflected when the focused spot crosses the guiding groove are inverted,is generated simultaneously and is added to the focus error signal. Thistransaction eventually means that a quantity, which is obtained byshifting phase φ by π and thus by inverting a sign of the first sin inEquation (21), is added, and accordingly the disturbance is canceledout.

[0096] The difference in reflectance between the guiding groove and theguiding inter-groove has required the adjustment of the gain with theuse of, for example, the width of the guiding groove. For instance, theabove-described adjustment has become necessary for the canceling of thetracking error signal off-set in the differential push-pull method.However, in the canceling of the disturbance mixed into the focus errorsignal, the gain adjustment is unnecessary.

[0097]FIG. 7 shows another embodiment for simultaneously detecting thelight beam in which the polarities of intensity distribution variationsof the reflected light beam reflected when the focused spot on theoptical disk crosses the guiding groove are inverted. In thisembodiment, a linear diffraction grating 701, which is located inparallel to a radial direction of the optical disk, is employed.Consequently, it turns out that the ±1st order diffracted lights formedby the diffraction grating on the optical disk are located on the sametrack as the 0th order light. Also, consequently, three four-dividedoptical detection regions 112, 113, 114 constituting an optical detector702 for detecting the reflected light beam are located in parallel to atangential direction of the optical disk.

[0098] Next, using FIG. 8, the description will be given concerning adetailed structure of the diffraction grating 701 employed in thepresent embodiment. The diffraction grating is constituted so that, asillustrated in FIG. 8, phase of the gratings is inverted with a periodof Dλ/(2NA·P) with reference to P, i.e. a period of the guiding grooves,NA, i.e. numerical aperture of the objective lens, and D, i.e. aneffective light beam diameter for a diffraction grating-insertedposition. This period is equal to an interval which is determined byshifts of reflected light beams of ±1st order diffracted lights 802, 803toward a 0th order light 801 of a diffracted light formed by the guidinggrooves of the optical disk. In a diffracted light formed by this kindof diffraction grating, phase of a wave surface of the diffracted lightis shifted by an amount of π for each period. Remembering that adiffraction grating is, originally, a hologram, this phenomenon can beunderstood easily. The hologram is produced by performing, on a filmsuch as a photographic dry plate, exposure and development processingsof an interference fringe formed by two high coherent lights such aslaser lights. When the hologram is irradiated with one of the lights atthe time of performing the exposure processing thereto, the other lightis reproduced as a diffracted light by means of the hologram. Then, asdescribed above, if the interference fringe is formed by causing aninterference to occur between the light the wave surface of which isshifted periodically by one-half of the wavelength and the light thewave surface of which is flat, it is quite natural that the interferencefringe should reflect the phase shift and discontinuously form a step ofone-half of the fringe. Accordingly, if, conversely, the light the wavesurface of which is flat is launched into such a diffraction grating, itturns out that wave surface of the diffracted light is shiftedperiodically by one-half of the wavelength.

[0099]FIG. 9 is a diagram for explaining a manner in which, when adiffracted light formed by the phase-inverted diffraction grating isfurther diffracted by the guiding grooves of the optical disk, phaseshift regions of the resultant diffracted light are overlapped. Thediffracted light by means of the phase-inverted diffraction grating isfurther diffracted by the guiding grooves of the optical disk, and the0th order light and the ±1st order diffracted lights are overlapped witheach other. However, between the diffracted lights which are adjacent toeach other, such as the 0th order light and the ±1st order diffractedlights, the phase shift regions are in contact with each other withoutbeing-overlapped. FIG. 10 summarizes phase differences added by thephase-inverted diffraction grating at this time between any two ofdiffraction orders included in each of the regions indicated by a, b, c,. . . in FIG. 9. FIG. 10 shows that phase differences between the lightswhich have adjacent diffraction orders and make a contribution to thetracking error signal, such as the 0th order light and the ±1st orderdiffracted lights, are equal to π without exception. Moreover, phasedifferences between the lights the difference in the diffraction ordersof which is equal to 2, such as the 0th order light and the ±2nd orderdiffracted lights, are equal to 0. Accordingly, concerning the phasedifferences in the interference shown in the Equation (5), withoutcausing the sub-spots to be off-track by one-half of the track pitch, itis possible to embody inversion of the interference intensities which isequivalent thereto. This transaction, even if storage marks existasymmetrically on the both sides of the central spot, brings about noasymmetry in an amount of reflected light of the sub-spots.Consequently, it becomes possible to further stabilize the effect ofcanceling the off-set in the tracking error signal or the effect ofcanceling the disturbance into the focus error signal.

[0100] Here, the phase-inverted diffraction grating is not integratedwith the objective lens. Accordingly, it turns out that, when theobjective lens moves following the decentering of the optical disk, anoptical axis of the phase-inverted diffraction grating and that of theobjective lens are relatively shifted with each other. FIG. 11, whichindicates the phase shift regions in this case, shows that the movementof the objective lens, even if it occurs, results in a mere movement ofconnected portions between the phase shift regions, thus bringing aboutno obstacle to the inversion of the interference intensities.

[0101]FIG. 12 shows still another embodiment for simultaneouslydetecting the light beam in which the polarities of intensitydistribution variations of the reflected light beam reflected when thefocused spot on the optical disk crosses the guiding groove areinverted. Here, instead of the phase-inverted diffraction grating inFIG. 7, a polarizing phase shifter 1201 is employed. The polarizingphase shifter 1201 inverts a phase of only a linearly polarized lightcomponent, which is polarized in a specific direction and launched intothe polarizing phase shifter, in regions of a period of λD/(2NA·P).Then, the linearly polarized light component is split and detected justin front of an optical detector 702, using a 3-beam Wollaston prism1202. At this time, unlike the case of the phase-inverted diffractiongrating, there occurs no sub-spots, and thus there exists only oneoptical spot on an optical disk 107. This condition makes it possible toreduce a loss in the amount of light caused by the sub-spots, thus beingable to constitute an optical head suitable for the writable opticaldisks.

[0102]FIG. 13 is a diagram for explaining the principle of thepolarizing phase shifter. Here, an example using lithium niobate(LiNbO₃) is presented. A lithium niobate substrate 1301 has a principalaxis 1302 having a refractive index anisotropy in the directionindicated in FIG. 13. Then, proton exchanged regions 1303 are formed inthe substrate in accordance with grating patterns. Moreover, inaccordance with the grating patterns, dielectric material layers 1304are formed. At this time, a phase difference between ordinary rays 1305,1306 which are each launched into the grating patterns and therebetween,and a phase difference Φ_(e) between extraordinary rays 1307, 1308 whichare each launched into the grating patterns and therebetween, arerepresented as the following equations, respectively: $\begin{matrix}{{\varphi_{o} = {\frac{2\pi}{\lambda}\left\{ {{\left( {n_{d} - 1} \right)T_{d}} + {\Delta \quad n_{o}T_{p}}} \right\}}}{\varphi_{e} = {\frac{2\pi}{\lambda}\left\{ {{\left( {n_{d} - 1} \right)T_{d}} + {\Delta \quad n_{e}T_{p}}} \right\}}}} & (22)\end{matrix}$

[0103] λ: Wavelength

[0104] n_(d): Refractive index of dielectric material layer

[0105] T_(d): Thickness of dielectric material layer

[0106] Δn_(o): Change of the ordinary refractive index of lithiumniobate by proton exchange (=−0.04)

[0107] Δn_(e): Change of the extraordinary refractive index of lithiumniobate by proton exchange (=0.12)

[0108] T_(p): Depth of proton exchanged region

[0109] Here, with the diffraction efficiency taken into consideration,setting the respective phase differences to be appropriate design valuesand solving Equation (22) as simultaneous linear equations with T_(d),i.e. a thickness of the dielectric material layers, and T_(p), i.e. adepth of the proton exchanged regions, as the unknowns, the solutionsare represented by Equation (23): $\begin{matrix}\begin{matrix}{T_{p} = {\frac{\lambda}{2\pi}\frac{\varphi_{o} - \varphi_{e}}{{\Delta \quad n_{o}} - {\Delta \quad n_{e}}}}} \\{T_{d} = {\frac{\lambda}{2\pi}\frac{{\Delta \quad n_{o}\varphi_{e}} - {\Delta \quad n_{e}\varphi_{o}}}{\left( {n_{d} - 1} \right)\left( {{\Delta \quad n_{o}} - {\Delta \quad n_{e}}} \right)}}}\end{matrix} & (23)\end{matrix}$

[0110] , which means that it is possible to design a polarizing gratingwhich allows desirable phase differences to be created concerning theordinary rays and the extraordinary rays independently of each other.For example, if the desirable result is that: the wavelength λ=0. 66 μm,the refractive index of the dielectric material layers n_(d)=2. 2,Φ_(o)=0°, and Φ_(e)=+180°, it will do to let T_(p)=2. 06 μm andT_(d)=0.07 μm. Taking the values in this way makes it possible toselectively shift the phase concerning the proton exchanged regions byan amount of π, thus being able to expect the same offset cancelingeffect as that in the above-described embodiments.

[0111] The description will be given below concerning a canceling effectobtained by a scalar diffraction simulation toward an disturbance in afocus error detection signal when a focused spot crosses the guidinggroove and a canceling effect obtained by the scalar diffractionsimulation toward an off-set which accompanies the movement of theobjective lens. FIG. 14 shows a focus error signal at the time whenthere exist the astigmatism, a spherical aberration and detectordeviations in a complex state in the ordinary focus error detectingsystem. The central portion is swelled, which demonstrates that thereoccurs a considerably large disturbance. On the other hand, FIG. 15shows a calculation result on the assumption that the phase-inverteddiffraction grating is used. This demonstrates that almost all of thedisturbance is canceled out. FIG. 16 shows a focus error signal obtainedby adding focus error signals of the main-spot and of the sub-spotsaccording to the differential push-pull method. This demonstrates thatalmost all of the disturbance can be canceled out.

[0112]FIG. 17 shows a case in which, with the objective lens being movedand in the ordinary astigmatic focus error detecting method, a trackingerror signal on the light-receiving surface is calculated. Thisdemonstrates that there occurs a considerably large off-set. Meanwhile,FIG. 18 shows a case in which the same calculation is performed usingthe phase-inverted diffraction grating. This demonstrates that almostall of the off-set is canceled out. Furthermore, FIG. 19 shows anembodiment in which the same calculation is applied in the differentialpush-pull method. This demonstrates that the off-set is made extremelysmall.

[0113]FIG. 20 is a diagram for showing an embodiment of an opticalsystem constitution according to the present invention in which thereproduction is possible especially in a CD, a CD-R, a DVD-ROM, and aDVD-RAM. Two semiconductor lasers, i.e. a 650 nm semiconductor laser2001 for the DVDs and a 780 nm semiconductor laser 2002 for the CD andthe CD-R, are mounted. In view of spectroscopic characteristics ofreflectance of a CD-R storage film, the 780 nm semiconductor laser 2002is absolutely necessary for the reproduction in the CD-R. The respectivelights are launched into diffraction gratings 2003, 2004, respectively,thus generating the ±1st order diffracted lights. Here, the diffractiongrating 2003 for 650 nm wavelength is a diffraction grating described upto now in the present invention, and the diffraction grating 2004 for780 nm wavelength is a diffraction grating for forming sub-spots for a3-beam tracking method usually employed to detect a tracking in the CD.Then, the light with 650 nm wavelength is reflected at a dichromaticmirror 2005, passes through a beam splitter 2006, is reflected at atriangle reflection mirror 2007, and is converged on the DVD 2009 by aDVD/CD compatible objective lens 2008. Meanwhile, the light with 780 nmwavelength is reflected at the beam splitter 2006 and at the trianglereflection mirror 2007, and is converged on the CD or the CD-R disk 2009by the DVD/CD compatible-objective lens 2008. Respective reflectedlights, by way of the DVD/CD compatible objective lens 2008 and thetriangle reflection mirror 2007, passes through the beam splitter 2006,the dichromatic mirror 2005 and an optical component G, then beingconverged on an optical detector 2010.

[0114]FIG. 21 is a diagram for explaining the optical component G, alight-receiving pattern constitution of the optical detector, and signalcalculation methods in a plurality of embodiments obtained by changingthe focal point shift detecting method in the above-mentioned opticalsystem constitution. When a beam size detection method is employed asthe focal point shift detecting method, a curvilinear diffractiongrating 2101 is employed as the optical component G. The curvilineardiffraction grating 2101, for each of a 0th order light and ±1st orderdiffracted lights generated by the diffraction grating 2003 or 2004,outputs optical spots to be situated somewhat before a focal point onthe optical detector surface and optical spots to be situated somewhatbehind the focal point thereon as ±1st order diffracted lights generatedby the curvilinear diffraction grating 2101. At this time, diffractionefficiency of the curvilinear diffraction grating 2101 is made largeenough. This prevents the 0th order light from being generated, thusmaking it possible to decrease the number of the detection regions. Inthis way, out of the six optical spots in total, from a set of the ±1storder diffracted lights generated by the curvilinear diffraction grating2101, a focal point shift error signal according to the beam sizedetection method is obtained. One of the 0th order lights generated bythe diffraction gratings 2003 and 2004 is received by a four-dividedoptical detector, thereby being able to obtain a DPD signal(differential phase detection) employed in the DVD-ROM. Also, it ispossible to obtain a push-pull signal in which the off-set is canceledout from one of the 0th order light and the ±1st order diffracted lightsgenerated by the diffraction grating 2003. It is possible to detect the3-beam tracking error signal for the CD from a difference in the amountof light between the ±1st order diffracted lights generated by thediffraction grating 2004. Also, it is possible to obtain a reproduced RFsignal from a total amount of light of the 0th order lights generated bythe diffraction gratings 2003 and 2004.

[0115] When a double knife edge method is employed as the focal pointshift detecting method, a light dividing prism 2102 is employed as theoptical component G. The light dividing prism 2102 divides each of thediffracted lights, which are generated by the diffraction grating 2003or 2004, into four lights on the optical detector surface. From the fourlights of one of the diffracted lights, a focal point shift error signalaccording to the double knife edge method is obtained. The signals, suchas the tracking error signal according to the push-pull method, the DPDsignal, the tracking error signal according to the 3-beam method, andthe RF signal, can be obtained, as shown in FIG. 21, in almost the sameway as in the beam size detection method.

[0116] In these focal point shift detecting methods, selection of thedirection of the divided lines in the optical detector makes it possibleto comparatively suppress, in the deviations and the aberrations, too,the occurrence of the disturbance which accompanies the crossing overthe guiding groove. Accordingly, in the present embodiment, theconstitution of adding the light the variations in intensitydistribution of which are inverted is not presented in particular.However, depending on the constitution of the optical system, it maybecome necessary from the other requirements to provide a constitutionof the divided lines in which the disturbance occurs easily. In thatcase, focus error signals of lights the variations in intensitydistribution of which are inverted are added to each other, therebyallowing the disturbance to be reduced in the focus error detectingmethods other than the astigmatic focal point shift detecting method,too. Consequently, the present invention also makes possible an opticalsystem constitution which, conventionally, could not be employed fromthe viewpoint of the disturbance which accompanies the crossing over theguiding groove. This characteristic allows a flexibility in the designto be increased.

[0117] When the astigmatic focal point shift detecting method isemployed as the focal point shift detecting method, the opticalcomponent G is unnecessary. The reason is that an astigmatism whichoccurs when the lights pass through the dichromatic mirror can besubstituted for the astigmatism for the astigmatic focal point shiftdetection. This is based on a principle that, when a focused light islaunched into a parallel flat plate, an astigmatism occurs. Here, in thefocus error detection, as described up to now from the viewpoint of thedisturbance, the focus error signals of the 0th order light and the ±1storder diffracted lights are added to each other. Conventionally, whenthe astigmatism is introduced using the parallel flat plate, theparallel flat plate was inserted in such a manner as to form an angle of45 degrees toward the tracks so that the disturbance which accompaniesthe crossing over the guiding groove does not occur easily. This kind ofrestriction, however, becomes unnecessary because of the canceling ofthe disturbance based on the present invention. Accordingly, in somecases, the present invention can be effective in making the optical headcompact in the whole size. Also, concerning the tracking error signalaccording to the push-pull method, the tracking error signal thedistribution of which is inverted is similarly subtracted. The othertransactions are performed in much the same way as in the cases in whichthe other focus error detecting methods are employed.

[0118] According to the present invention, by adding an inexpensivecomponent such as a diffraction grating to a fixed optical systemwithout mounting it on the objective lens actuator, it is possible tofundamentally eliminate the disturbance which occurs in the focus errorsignal in association with the decentering of an optical disk when anoptical spot crosses a track on the surface of the storage film. At thesame time, it is possible to fundamentally cancel the off-set whichoccurs in the tracking error signal in association with the movement ofthe lens.

What is claimed is:
 1. An information reproducing method for reproducinginformation recorded on a medium by irradiating a laser beam anddetecting a reflected light beam from the medium, comprising the stepsof: detecting a plurality of reflected light beams in which thepolarities of variations in reflected light beam intensity distributionsare substantially inverted to each other, the distribution variationbeing produced when a light spot of said laser beam crosses a track onsaid medium; adding focus error signals for said plurality of reflectedlight beams to generate a focus error signal; and taking a differencebetween tracking error signals of said plurality of reflected lightbeams to generate a tracking error signal.
 2. An information reproducingmethod according to claim 1, wherein said medium has a groove widthequal to approximately a half of a track pitch.
 3. An informationreproducing method according to claim 1, wherein said tracking errorsignal is obtained by amplifying each of tracking error signals for saidplurality of reflected light beams with a gain proportional to a ratioof reciprocal of a total amount of each reflected light beam and thenoperating a difference between the amplified tracking error signals. 4.An information reproducing method according to claim 1, wherein a lightreflection when said light spot is in said groove is substantially equalto that when said light spot is between grooves.